Wafer handling apparatuses, e.g., heaters and electrostatic chucks, are used in a number of system applications such as molecular beam epitaxy, space experiments, substrate heaters for electron microscopy and in the growth of superconducting films, etc. Heaters are typically used to heat a semiconductor wafer in the manufacture of semiconductors. A wafer handling assembly may include a susceptor for supporting a wafer, and a plurality of heaters disposed under the susceptor to heat the wafer. The semiconductor wafer is heated within a confined environment in a processing vessel at relatively high temperature and often in an atmosphere, which is highly corrosive. The temperature to which the wafer is heated is also controlled to within a predetermined range after reaching a desired processing temperature. Heretofore, the heating device typically consists of a heating platen formed of a sintered ceramic body in which a refractory metal wire was embedded. The refractory metal wire operates as a heat generating resistive element when connected to an external power supply. However, because the resistive element is embedded in a ceramic material the amount of power the heating device can generate and deliver to the wafer is limited which limits the amount of power available.
Pyrolytic boron nitride (PBN) is formed by chemical vapor deposition of boron nitride in a reactor chamber by the vapor phase reaction of ammonia and a boron containing gas such as boron trichloride (BCl3). The pyrolytic boron nitride is of very high purity and when separated or released from the substrate forms a self-standing article of purified pyrolytic boron nitride. In other cases, the pyrolytic boron nitride coating can be made to adhere to the substrate to form a coated article.
In the prior art, heaters typically include a dielectric base of boron nitride and a heating element formed from a conductive material capable of resistive heating such as graphite and more particularly pyrolytic graphite. The heating element is connected to an external power supply to form a resistive heater. In certain applications such as in the growth of superconducting films, it is necessary to introduce oxygen into the atmosphere of the reacting chamber in which the superconducting film is grown. The oxygen in the atmosphere will react with any exposed graphite conductor in the heating unit to oxidize the conductor causing an open circuit.
U.S. Pat. No. 5,343,022 discloses a pyrolytic heating element in which multiple graphite post (or shaft) connectors are used. The shafts have internal tapped holes for attachment to an external power supply. The assembled heating element and shafts are then coated with a pyrolytic boron nitride layer to encapsulation the conductor and shafts to isolate the graphite from the process chemistry. In this design with multiple electrical leads in separate posts, the mechanical connection around the leads tend to increase stress in thermal expansion during operation thus often breaking the heating element or the leads. In some applications, the thermal stress of the installation can cause an arc at the point of electrical contact with the heating element which will damage the heating unit and render it nonfunctional.
U.S. Pat. No. 6,066,836 discloses a heating structure having a resistive heating substrate holder including a support surface (wafer holder) and a support shaft comprising a relatively pure compound of aluminium nitride (AlN), wherein the support shaft is diffusion bonded to the wafer holder.
Applicants have also discovered a wafer processing apparatus, e.g., a heating device, employing a unitary construction, e.g., an assembly of a graphite support plate containing CVD (chemical vapor deposition) film of pyrolytic graphite as the active heating element connected through conductors to an outside source of electrical power to heat the semiconductor wafer, and a graphite shaft containing the electrical conductors and protecting them from exposure to the hostile corrosive atmosphere within the vessel used to process the semiconductor wafer. The use of graphite as opposed to AlN in the prior art, surprisingly improves the performance of the heating structure in the manufacture of semiconductor wafers.
The present invention further relates to a novel design of an electrical connection assembly for a pyrolytic heating element having improved mechanical strength and extended life in operations.